Apple's iOS 5 is currently jailbreakable, but only with a tethered jailbreak. That means if your phone ever crashes, or if you have to reboot it for any reason, you need to have it connected to a Mac or PC. That sucks! If someone ever wants to murder you they'd just reboot your phone and you'd have no way to call the cops. But ho! SemiTether solves this problem.

SemiTether is a Cydia download that bypasses the rebooting limitation, allowing you to get back to your phone without connecting it to a computer. However, you will not be able to use any jailbreak apps, or use Mail and Safari, and your phone might take longer to boot than usual. If you compare this to having an entirely unusable phone, it's a pretty decent alternative.